This invention relates to shell and tube heat exchangers and similar equipment and is particularly concerned with improving of the arrangement for sealing a pass baffle in a manner which prevents fluid leaks which have the effect of bypassing within the heat exchanger.
Shell and tube heat exchangers are widely used for the indirect transfer of heat from one fluid to another. Typically, such an exchanger consists of an external shell having inlet and outlet ports for circulation of the shell-side fluid. A bundle of tubes is positioned within the shell and provided with transverse baffles or cross baffles for directing the shell-side fluid back and forth across the tubes. The tubes are supported by one or more tube sheets, one of which is normally stationary, and if another is used, it may be of the floating type to accommodate changes in tube length due to thermal expansion. The tube bundle and shell may be arranged so that the tube-side fluid makes a single pass through the shell or instead makes two or more passes. In a single pass exchanger, the tube-side fluid is introduced into a head at one end of the shell and withdrawn from a second head at the other end. In a multiple pass unit, the exchanger will generally be provided with an internal head containing one or more baffles so that the tube-side fluid can be introduced into one portion of the head and withdrawn from the other portion. An internal head within which the tube-side fluid flows from one set of tubes into another will generally be located at the other end of the tube bundle. A further type heat exchanger has an integral cover head which comprises two parts where the tube-side fluid is introduced in one part and extracted in the other part. The tube bundle comprises U-tubes which are arranged in a two pass shell.
Those experienced in the art will readily appreciate that a wide variety of different combinations of shell and tube arrangements may be employed as the process requirements demand. However, in all such arrangements, it is generally desirable to have efficient and effective fluid seals between the shell and any pass baffles to prevent bypassing of inlet fluid around the pass baffle to the shell outlet connection, thus losing efficiency of the heat exchanger. Reference may be made to applicant's two prior patents, U.S. Pat. Nos. 3,958,630 and 4,142,578, for background in the heat-exchange field, and their disclosure is hereby incorporated herein by reference.
The present invention is particularly applicable to process heat exchangers with removable tube bundles and multiple shell-side passes such as more completely described in The Tubular Exchanger Manufacturer's Association (TEMA), Type BFU, such industry standards being herein incorporated by reference. While this general type industry standard heat exchanger offers very desirable process flexibility, it has not always proved effective in service because the pass baffle/shell seals are ineffective and much of the shell-side fluid can bypass the tube bundle, particularly after maintenance has been done to the tube bundle and the pass baffle seals damaged. In heat exchangers of this type, the higher pressure is always in the inlet of the shell and the lower pressure is at the outlet of the shell. The differential pressure across the pass baffle aids in the sealing of the pass baffle to the shell wall, but what has been experienced is that the commercially available seal designs, while initially being effective (having been assembled by the manufacturer with great care), are easily damaged in the field during maintenance operation on the tube bundle. Typically, such maintenance involves cleaning of scale from the tubes requiring that the bundle must be removed from the shell, lifted onto pallets, rolled over for cleaning, lifted again, and reinserted into the shell. At best, bundles are lifted with wide straps that pass around the lower periphery of each bundle and very readily damage the existing seals of such bundles installed by the manufacturer since they normally protrude beyond the periphery of the bundle and therefore are in contact with the straps during any lifting or rolling operations.
The problem of seals between the internal parts of heat exchangers has been addressed by numerous prior art patents; for example, U.S. Pat. No. 2,550,725 shows a heat exchanger employing elongated spring steel strips for locating and securing the pass baffle relative to the exchanger shell. Another U.S. Pat. No. 2,900,173 also contemplates an arcuate or curved seal strip located in a notch in the head wall to seal against a baffle plate. Other patents such as U.S. Pat. No. 1,955,006 shows an arrangement for injecting a lubricant into the gap between a baffle plate and the shell wall of a heat exchanger.
While all of these references recognize the desirability of sealing within heat exchangers, none of them solves the problem according to the present novel and advantageous design.